Very spur of the moment, but turned into one of my fondest memories.
One of the most memorable places I've masturbated was sitting in the umpires chair on a tennis court in the middle of a big tennis complex, watching my friends on our team play in a state tournament. I was up there umpiring, but the unspoken rule was that everyone hated umpiring so we'd just sit up there and watch so the organizers didn't get upset while those playing umpired themselves.
So, under 16s girls, I was sitting up there bored and I had one of those 'wouldn't it be funny if' moments and decided to see if I could rub myself up there without anyone noticing so I pulled my legs up to the seat, crossed at the ankles and knees resting on the arm rests, rested my hand in my lap and started stroking my leg under my skirt, and then up and down my lips through my bikeshorts. Not hard or fast, just moving a couple of fingertips back and forth.
It didn't take long before I felt my clit perking up and realising I was actually doing it right there in the middle of about 30 courts of people playing tennis and no one could see or cared what I was doing was really really hot, so I just went for it to see if I could actually have an orgasm right there. I made myself comfortable, pulled the back of my skirt up (just in case..), my legs as wide as I could sitting in that chair and just moving the tips of a couple of my fingers I started rubbing over my clit thru my pants, still slow and regular but firmer, up and down and moving it side to side.
The whole situation of what I was doing got me right up there so fast, but the slow and regular rhythym I was making myself go at because I was still scared of getting caught made that last big rise before going ever so slow and drawn out it was crazy. And then I was there, and then I went over and just hung on, still with the same steady pace but digging my fingers in hard as I peaked and tried not to grunt and jerk which I still did a little bit. It was so intense it took me a while to refocus and then everyone was still playing. The match I was watching was in the middle of another rally. It was the most unique feeling ever; coming down from a massive orgasm and the whole world going on like nothing happened. The whole experience made me feel so naughty and horny I could have easily done it a bunch more times, but the set was getting near the end and I had to play next, so I just kept up a gentle stroking until the end of the set, and had a few little minigasms and twitches until I had to climb down and start playing my set a bit damp and weak-legged.